BUG#15024: get_best_covering_ror_intersect() tries to build ROR-intersection

by starting with an empty index set and adding indexes to it until it
becomes covering. If the set becomes covering after adding the first index,
return NULL and don't try constructing ROR-intersection of one index (which
caused a crash)

sql/opt_range.cc

@@ -3136,10 +3136,10 @@ TRP_ROR_INTERSECT *get_best_covering_ror_intersect(PARAM *param,
     /* F=F-covered by first(I) */
     bitmap_union(&covered_fields, &(*ror_scan_mark)->covered_fields);
     all_covered= bitmap_is_subset(&param->needed_fields, &covered_fields);
-  } while (!all_covered && (++ror_scan_mark < ror_scans_end));
-
-  if (!all_covered)
-    DBUG_RETURN(NULL); /* should not happen actually */
+  } while ((++ror_scan_mark < ror_scans_end) && !all_covered);
+  
+  if (!all_covered || (ror_scan_mark - tree->ror_scans) == 1)
+    DBUG_RETURN(NULL);
 
   /*
     Ok, [tree->ror_scans .. ror_scan) holds covering index_intersection with